1. Field of the Invention
The present invention relates to an electroacoustic transducer which generates a sound by means of electromagnetic acoustic conversion.
2. Description of the Related Art
FIG. 4 is a perspective view showing an example of an electroacoustic transducer of the conventional art. The electroacoustic transducer 90 comprises a diaphragm 93 made of a magnetic material; a magnetic piece 94 fixed to the center of the diaphragm 93; an electromagnetic coil (not shown) supplying an oscillating magnetic field to the diaphragm 93; a housing member 91 accommodating the diaphragm 93 and the electromagnetic coil; etc. Since a sound release opening 91b having a diameter larger than the magnetic piece 94 is formed in a top plate 91a of the housing member 91 to oppose to the diaphragm 93, such a transducer is usually called an open-type electroacoustic transducer.
When an electric oscillating signal is supplied to the electromagnetic coil, an oscillating magnetic field generated by the electromagnetic coil oscillates the diaphragm 93 to generate a sound. The sound is released to the outside through the sound release opening 91b. 
In the open-type electroacoustic transducer 90, ideal acoustic performance is attained by reducing sound interference. Therefore, conventionally nothing is placed in front of the diaphragm 93 to increase the area of the sound release opening 91b. 
When an operator or a working robot erroneously contacts with the diaphragm 93 during a process of packing or transportation of the transducer 90 or mounting the transducer on a circuit board, the diaphragm 93 is deformed or becomes dirty, and desired performance cannot be obtained. In order to enhance the oscillation efficiency, the diaphragm 93 is formed to be very thin, and hence a component failure is caused even by a light contact.
In order to miniaturize and lighten the transducer 90, moreover, the housing member 91 also is formed to be very thin. When the sound release opening 91b is large, therefore, the strength of the housing member itself is decreased so that the housing member 91 is deformed into an oval shape, simply by, for example, pressing the outer periphery by fingers. This causes a dimensional failure in a subsequent assembling step.
It is an object of the invention to provide an electroacoustic transducer in which contact with a diaphragm and deformation of a housing member can be surely prevented from occurring, and which can be miniaturized and produce a high sound pressure.
According to the invention, there is provided an electroacoustic transducer comprising:
a diaphragm made of a magnetic material having a magnetic piece fixed onto the center there of;
a magnetic core disposed with a gap Ga from the diaphragm;
an electromagnetic coil for supplying an oscillating magnetic field to the magnetic core; and
a housing member accommodating the diaphragm, the magnetic core, and the electromagnetic coil, and defining a sound release opening in a top surface thereof opposed to the diaphragm, the sound release opening being larger than the magnetic piece,
wherein a beam portion is integrally formed with the housing member, and extends from a peripheral portion of the sound release opening to another peripheral portion thereof;
the beam portion passes above the magnetic piece with a gap Gb being defined between the magnetic piece and the beam portion; and
the gap Ga and the gap Gb satisfy a relationship of Gaxe2x89xa6Gbxe2x89xa65Ga.
According to the invention, a beam portion which passes above the magnetic piece is formed in the sound release opening of the housing member, so that it is possible to surely prevent an operator or an object from erroneously contacting with the diaphragm. Therefore, the incidence of defective components of the transducer can be suppressed. In this case, preferably, the opening partitioned by the beam portion is smaller in dimension than an object which may possibly contact with the diaphragm.
Since the peripheral portion of the housing member and the beam portion are integrally formed, the strength of the housing member can be enhanced, so that deformation and a dimensional failure of the housing member can be prevented from occurring.
Even when the beam portion is configured by a thin member, the beam portion can exert sufficient strength. Therefore, the beam portion does not produce a large influence on acoustic performance.
In order to attain high magnetic coupling between the diaphragm and the magnetic core, it is preferable to set the gap Ga between the diaphragm and the magnetic core to be as small as possible. However, a distance of a predetermined value or larger must be ensured so as to prevent the diaphragm from being in contact with the magnetic core when the diaphragm is oscillated. With respect to the gap Gb between the magnetic piece and the beam portion, similarly, a distance of a predetermined value or larger must be ensured so as to prevent the magnetic piece from being in contact with the beam portion when the diaphragm is oscillated. When the gap Gb is set to be excessively large, the whole height of the transducer is increased more than necessary. Consequently, it is preferable to set the gap Gb to be equal to or larger than the gap Ga. In consideration of dimensional and mounting errors of the housing member, and the like, it is preferable to set the gap Gb to be equal to or smaller than five times the gap Ga.
According to the invention, a ratio Se/So is 70% or more, where So is an opening area of the sound release opening, and Se is an effective opening area eliminating an area of the beam portion.
According to the invention, since the ratio Se/So is 70% or more, the influence of the beam portion on the acoustic performance is negligibly small. From the viewpoints of prevention of contact with the diaphragm and enhancement of the housing member, it is preferable to increase the dimensions of the beam portion. However, the opening area is reduced by the beam portion, and hence there is fear that the acoustic performance may be different from that of a full-open-type transducer. By the tests to measure acoustic performance with respect to the size of the beam portion, it has been confirmed that, when Se/So is 70% or more, acoustic performance is not substantially different. Consequently, interchangeability of the electroacoustic transducer of the invention and a full-open-type transducer can be maintained.